Israel blocked around 200 pro-Palestinian activists from boarding European flights to the country, aided by Facebook. It's hard to pretend to be a tourist in the era of social networking.

Many activists never even made it to the airport, the AP reports, because airlines called them at home to say Israeli authorities had blacklisted them. Others were turned away at check-in counters. Some activists tried to pose as tourists—"I said I was coming to visit. That was it," one told the newswire—but Israeli security monitors activists activity on Facebook closely, and compiled a list with 300 names on it, which it circulated to its embassies and in turn to airlines.

The lesson: Do your online activism under an assumed name. Perhaps of someone you yourself would like to prevent from going to Israel.